Using a knife with a foldable or sliding blade is generally a two-handed operation, and most knives that are operable with one hand are illegal in many states.
One-handed operation is desirable for speed and/or safety for workers or sportsmen who often use knives and must quickly cut open boxes, cut line, and the like. Fixed blade knives are simply not well suited for this purpose in all cases. Pocket knives, attached to a key chain or the like, are useful to some extent in that they are readily available for use, but as stated above, they require two hands to deploy the blade and moreover generally offer no means to quickly separate the knife from the chain should use of the knife free of its attaching chain be desired.